The present invention concerns hybrid layered cables some of which can be used to reinforce at least one crown protection ply for tyres fitted on heavy vehicles or earthmovers, and others to reinforce the tyre beads in light motorised vehicles such as motorcycles. The invention also concerns a composite fabric usable as a crown protection ply for such heavy vehicles or earthmovers, a bead wire intended to reinforce the said tyre beads, and the aforesaid tyres.
As a general rule steel cables for tyres consist of wires of pearlitic (or ferrite-pearlite) carbon steel, hereinafter “carbon steel”, with a carbon content generally between 0.2% and 1.2%, the diameter of the wires being most often between 0.10 and 0.40 mm. These wires are required to have very high tensile strength, generally higher than 2000 MPa and preferably higher than 2500 MPa, obtained thanks to the structural hardening that takes place during the cold-drawing of the wires. The wires are then assembled in the form of cables or strands, which requires the steels used also to possess a torsional ductility sufficient to withstand the various cabling operations.
In a known way tyres for heavy vehicles, earthmovers and light motorised vehicles usually comprise a carcass reinforcement anchored in two beads and surmounted radially by a crown reinforcement comprising one or more working crown plies, this crown reinforcement itself being surmounted by a tread joined to the tyre beads by two sidewalls. For heavy vehicle or earthmover tyres only, the crown reinforcement also comprises one or more crown protection plies surmounting the working crown ply or plies.
The crown protection plies essentially have the function of presenting an obstacle, during rolling, to the penetration of foreign bodies radially into them, foreign bodies which affect earthmover tyres in particular, since they often travel over ground covered with sharp stones.
To reinforce the carcass reinforcement and the working crown ply of radial tyres such as heavy vehicle tyres, one usually uses steel cables said to be “layered” or “multilayered”, which consist of a central core and one or more concentric layers of filaments arranged around the core, so that the rigidity of the cable will be almost equal to the sum of the rigidities of the filaments constituting it. Among layered cables a distinction is made in particular, in a known way, between cables with a compact structure and cables with tubular or cylindrical layers.
Such layered cables have been described in a very large number of publications. Reference can be made in particular to the documents GB-A-2 080 845; U.S. Pat. Nos. 3,922,841; 4,158,946; 4,488,587; EP-A-168 858; EP-A-176 139 or U.S. Pat. No. 4,651,513; EP-A-194 011; EP-A-260 556 or U.S. Pat. Nos. 4,756,151; 4,781,016; EP-A-362 570; EP-A-497 612 or U.S. Pat. No. 5,285,836; EP-A-567 334 or U.S. Pat. No. 5,661,965; EP-A-568 271; EP-A-648 891; EP-A-601 402 or U.S. Pat. No. 5,561,974; EP-A-669 421 or U.S. Pat. No. 5,595,057; EP-A-675 223; EP-A-709 236 or U.S. Pat. No. 5,836,145; EP-A-719 889 or U.S. Pat. No. 5,697,204; EP-A-744 490 or U.S. Pat. No. 5,806,296; EP-A-779 390 or U.S. Pat. No. 5,802,829; EP-A-834 613 or U.S. Pat. No. 6,102,095; WO-A-98/41682; RD (Research Disclosure) No. 316107, August 1990, p. 681; RD No. 34054, August 1992, pp. 624-33; RD No. 34370, November 1992, pp. 857-59; RD No. 34779, March 1993, pp. 213-214; RD No. 34984, May 1993, pp. 333-334; RD No. 36329, July 1994, pp. 359-365.
The layered cables most widely used in the carcass reinforcements and working crown plies of radial tyres are essentially cables having the formulae [M+N] or [M+N+P], the latter generally intended for the largest tyres. These cables are formed in a known way from a core of M filament(s) surrounded by at least one layer of N filaments, if necessary itself surrounded by an external layer of P filaments, in which M generally ranges from 1 to 4, N from 3 to 12 and P from 8 to 20, depending on the case, the whole if necessary being hooped by an external hoop filament rolled in helix around the last layer.
American patent U.S. Pat. No. 4,176,705 discloses a hybrid layered cable designed in particular to reinforce the carcass reinforcement of a heavy vehicle or earthmover tyre. The inner layer of this cable consists of a multifilament core made of a non-metallic material with tensile strength equivalent to that of steel, and the external layer of the cable, which is unsaturated, consists for example of 6 metallic strans each comprising 4 wires twisted in a direction opposite to that of the said strands, in a S-Z structure.
The non-metallic material constituting the said multifilament core, preferably aramid, is chosen specifically to provide the cable with a rigidity close to the sum of the rigidities of its constituents and not to reduce the overall tensile strength of the cable, in contrast to materials such as aliphatic polyesters whose tensile strength is far lower than that of steel. This aramid core is also provided to fill the gaps between the stranded wires, so as to minimise the corrosion of the cable due to water infiltration, and the diameter of the core is accordingly chosen equal to or larger than that of each metallic strand.
For the reinforcement of the crown protection plies of heavy vehicle or earthmover tyres, the cables generally used nowadays are not layered cables but cables with strands (“strand cables”) which are assembled by the known technique of stranding and which consist, by definition, of a plurality of metallic strands twisted together in helix, each strand comprising a plurality of steel wires also twisted together in helix.
Note that most of the filaments used in these cables for crown protection plies have a diameter typically larger than 0.20 mm, for example close to 0.25 mm, a diameter larger in particular than that of the filaments used in the cables for carcass reinforcements in the said tyres. These cables for crown protection plies are intended on the one hand to confer optimum suppleness on the ply containing them so that the said crown protection ply can the better conform to the shape of the obstacle it encounters during rolling, and on the other hand to allow the ply to resist the penetration of foreign bodies radially into it.
Note also that the said strand cables must be impregnated as completely as possible by the rubber, so that the latter penetrates into all the spaces between the wires constituting the cables. In effect, if penetration is insufficient, empty channels are formed along the cables and corrosive agents, for example water, which can penetrate into tyres for example after cuts or other damage to the crown reinforcement of the tyre, travel along these channels through the said reinforcement. The presence of this humidity plays an important part in causing corrosion and accelerating fatigue (so-termed “fatigue-corrosion” phenomena), compared with use in a dry atmosphere.
For the specific reinforcement of the crown protection plies of tyres for earthmovers, the applicant at present uses strand cables of formula 4×6 (i.e. consisting of 4 strands, each with 6 steel wires) in the earthmover tyres of size “40.00 R57 XDR” that it markets, each wire in a strand having a diameter for example of 0.26 mm. Experience has shown that these cables are entirely satisfactory for this reinforcing function, since in particular they delay the appearance and spread of perforations or cuts between the tread and the crown protection plies.